1. Field of the Invention
This invention relates to a solid state shear extrusion process and apparatus for continuous pulverization to fine particles of a wide variety of solids and their mixtures, such as, polymers, copolymers, homopolymers, agrowastes, rubber, wood chips, and mixtures of synthetic and natural polymers which have been resistant to such fine pulverization. The invention is particularly well suited for fine pulverization of thermoplastic polymers. The process and apparatus of this invention is especially useful for recycling of synthetic and natural polymers and mixed polymer wastes using a non-cryogenic and low power consumption technique.
2. Description of Related Art
Currently, three basic reclaiming processes of virgin and used plastics are practiced: Chemical, which include chemical treatment, as well as, pyrolysis, hydrolysis, and incineration; Physical, which include melting and subsequent extrusion, injection molding, and pressure molding; and Mechanical, which include granulation, densification, agglomeration, and pulverization. Presently used processes have disadvantages of high energy consumption, decrease in original properties of the polymers, applicability to only specific polymers, and environmental undesirability.
U.S. Pat. No. 4,090,670 teaches recovery of rubber from scrap vulcanized rubber tires by raising the surface temperature sufficiently to devulcanize followed by removal of the devulcanized material, such as by rasping. This method is limited to rubber and does not produce fine powders as desired for many reuse applications.
Reclamation of thermoplastic materials including shredding, grinding and comminuting is exemplified by: U.S. Pat. No. 4,968,463 teaching shredding waste plastic to about 100 mm and grinding to under about 40 mm, followed by drying preheating to 80.degree. to 160.degree. C., kneading at 120.degree. to 250.degree. C., and injection molding or extrusion; U.S. Pat. No. 4,650,126 teaching heating plastic particles to melt the surface to retain a grinding aid thereon and maintaining a counter-rotating attrition mill at a temperature to retain nearly all of the grinding aid on the softened polymer particles during grinding, followed by an air stream which serves to separate the grinding aid and as a material carrier medium; U.S. Pat. No. 4,511,091 teaching thermoplastic scrap recovery combined with phonograph record pressing wherein the hot trimmed waste is cooled, ground, and mixed with virgin material for formation of phonograph records; and U.S. Pat. No. 4,098,463 teaching a liquid cooling spray to maintain the temperature in a cutting chamber such that the plastic is hard which reduces the fibers imbedded in the comminuted particles from plastic electrical or telephone cord insulation.
Various screw devices are known for conveyance and processing in the synthetic polymer industry. Molding of products from a mixture of thermoplastic polymers or a thermoplastic polymer and an inorganic material by control of crystallization in a screw extruder with temperature control in a first portion within 35.degree. C. below the material melting point and the temperature in a second portion within 35.degree. C. above the material melting point with the maximum temperature at the outlet is taught by U.S. Pat. No. 5,026,512. U.S. Pat. No. 4,890,996 teaches continuous granulating by melting, kneading and granulating macromolecules wherein a double screw kneader without lateral communication is capable of adjusting the degree to which material is kneaded by axial adjustment of the cylinders and screws with respect to each other.
Conical screw sections are known to be used for specific purposes. U.S. Pat. No. 4,875,847 teaches a twin screw extruder, especially suited for viscous materials, having frusto-conical screw sections and separate barrel sections at the outlet end providing bearing-type support for the separate screws. U.S. Pat. No. 3,525,124 teaches an extracting apparatus having screw-threaded shafts rotatable within a housing and having conveying and milling sections with an obstruction section between for pressure sealing. The screw and the housing may be tapered to form the obstruction section, thereby providing independent heat and pressure control in the conveying and milling sections.
Chemical and physical aspects of transformations of polymeric materials, such as pulverization, under simultaneous high pressure and shear is described in Nikolai S. Enikolopian, "Some Aspects of Chemistry and Physics of Plastic Flow", Pure & Appl. Chem., Vol. 57, No 11, pp. 1707-1711, (1985).
U.S. Pat. No. 4,607,797 teaches pulverization of fused polymers in an extrusion apparatus having a barrel with at least one cylindrical rotatable screw. When two screws are used, they are co-rotational. The '797 patent teaches that material is fed to one end of the barrel, heated to above its fusing temperature in a first zone, cooled to below its solidification temperature with simultaneous pre-crushing and pulverizing of the solidified material in a second zone, and discharge of the powdered material from the opposite end of the barrel. Screw action is used to convey the material through the barrel and substantially elliptical or triangular kneading or pulverizing discs non-rotatably mounted on the screw in the cooling zone provide the pre-crushing and pulverizing. The process is carried out at 0.25 to 0.30 MPa. This process is said to continuously produce particles having a very uniform grain size, in the case of polyethylene only 2% larger than 160 microns. The '797 patent distinguishes its process from the prior art by allowing solidification of the fused material within the extrusion apparatus.
U.S. Pat. 4,607,796 teaches pulverization of rubber and vulcanization products in a standard single- or multiple-screw extruder by compressing at a pressure of 0.2 to 0.7 MPa, then subjecting the compressed material to a shear force of 0.03 to 5 N/mm.sup.2 at a pressure of 0.2 to 50 MPa and temperature of 80.degree. to 250.degree. C. forming hot sheared material which is subjected to a shearing force of 0.03 to 5 N/mm.sup.2 at a pressure of 0.2 to 50 MPa and temperature of 15.degree. to 60.degree. C. forming cooled powder material. Addition of granulated polyethylene to butyl rubbers is necessary to obtain finely dispersed powders. This process is said to result in particles not exceeding 500 micrometers in the case of natural rubber and 300 micrometers with other rubbers.